Apparatus for limiting the coating edge in a short dwell coater

ABSTRACT

A coating edge limiter for a coating arrangement with a short dwell time. Color is fed under pressure into a coating chamber formed by a backing roll around which a paper web is guided, a barrier wall, a doctor blade element and sealing blocks located at both ends of the coater. A formatting plate almost completely covers an application zone between the doctor blade element and the barrier wall so that an edge region of the web is sharply delineated and kept completely free of color. In each of the sealing blocks water is supplied via channels to depressions thereby forming a pressure cushion. This pressure cushion prevents the pressurized color in the coating chamber from flowing out laterally and ensures that the formatting plate is pressed flatly against the web. The coating edge limiter thereby makes it possible to establish very precisely the width of the web to be produced.

FIELD OF THE INVENTION

The invention relates to a coating edge limiter for a coatingarrangement with a short dwell time.

BACKGROUND OF THE INVENTION

A short dwell coater is disclosed in U.S. Pat. No. 4,250,211. Coatingedge limiters of the aforementioned type are known and used mainly assealing elements at both roll ends in order to prevent coating colorfrom flowing laterally out of a coating chamber.

An end seal for a coating arrangement with a short dwell time isdescribed in German Patent publication 38 15 764 A1. An upper outersurface of the end seal facing the paper web has openings through whichwater is introduced between the outer surface and the paper web for thepurpose of improving tightness and reducing wear on the end seal. Adisadvantage of this design is that the coating color diluted by theintroduced water leaves behind less coating on the edges of the paperweb thereby creating a transitional area in which the coating weightdeclines. A trimming edge of corresponding width must then be cut offbecause the machine operator cannot determine visually the exact pointup to which the paper web has the required coating weight. Furthermore,the water supply causes the dry web edges to become moist. This leads towaviness at the web edges, particularly in the case of lightweight paperwebs. The running qualities of the web deteriorate to the point that theweb tears. Using this known end seal, it is not possible to obtain aclean edge free of coating color.

SUMMARY OF THE INVENTION

The object of the present invention is to create coating edge limiterscapable of attaining sharply delineated web edges completely free ofcoating color while avoiding contamination with encrusted color.

This object is attained with the present inventive coating edge limiterfor a coating arrangement with a short dwell time, including a matingroll with a circumferential surface that carries a material web,especially a paper web. A doctor blade element extends over the entirewidth of the material web and is used to apply the coating. Anapplication zone is formed between the doctor blade element and abarrier wall oriented parallel to the doctor blade element. Two coatingedge limiters, each with a sealing block and a sliding element, arearranged in an edge region movable at a right angle to the webdirection. A formatting plate holder is connected permanently to thesealing block and extends outwardly over the barrier wall. On theformatting plate holder a thin formatting plate is arranged in such away as to extend from the outside over the barrier wall into the spacebetween the barrier wall and the doctor blade element in order tocompletely cover the application zone in the edge region of the materialweb. The sealing block, on the side facing the formatting plate, has atleast one depression, that extends parallel to the direction of webmovement and is connected to a water supply by a channel. A narrowsealing gap, from 1 to 3 mm wide, is formed between the formatting plateand the sealing block. Water introduced into the sealing gap through thedepression produces a pressure cushion and presses the formatting plateflatly against the material web. The formatting plate holder, preferablyformed of spring steel, has a thickness greater than or equal to 0.4 mmand is attached to the formatting plate holder by some type of clampingmeans. Furthermore, the formatting plate, at a right angle to thedirection of web movement, is of a width that overlaps the width of thesealing block by approximately 5% to 20%. Preferably, the sealing blockis formed of a metal material. A water spraying device is located at thefree end of the formatting plate holder. The water spraying device has aneedle nozzle through which a fine jet of water is sprayed onto theformatting plate.

The formatting plate permanently connected to the coating edge limitercompletely covers the application zone in the edge area of the web. Thecontinuous supply of water in the sealing nip between the formattingplate and the sealing block creates a pressure cushion. This pressurecushion prevents the pressurized color in the coating chamber fromflowing out laterally and also keeps the formatting plate pressed flatlyagainst the web. As a result, no coating color can tie applied to theweb where the web is covered by the formatting plate. In this way, asharp delineation is achieved between the coated material and the edgestrips to be trimmed. In other words, in accordance with the coatingedge limiter of the present invention it is possible to establish veryprecisely the width of the web to be produced.

The formatting plate, preferably made of spring steel having a thicknessof a few tenths of a millimeter, preferably less than or equal to 0.4mm, adapts well to the curved contour of the backing roll. Theapplication zone is thus covered until directly in front of the doctorblade element. In contrast, the sealing block, which is advantageouslyoverlapped by the formatting plate by 5% to 20%, has a simple contourthat does not need to be exactly adapted to a more or less curved doctorblade element. The emergence of color is prevented, as alreadydescribed, by the pressure cushion which forms as the supplied waterbuilds up in the sealing nip. Because the surface of the formattingplate remains at rest, in contrast to the moving material surface, theflow speed of the color declines sharply there. The color mixes with thesupplied water to only a slight extent. Moveover, the continuous outwardflow of water from the sealing nip prevents disruptive deposits ofcoating, to thereby keep the coating edge limiter clean.

In an embodiment of the invention, the sealing block is made of metal,particularly rust-free steel. Because of the good heat conductivity ofmetals, condensation water forms on the surface of the sealing block,which remains relatively cold due to the continuous flow of cold water.For this reason, no color deposits can accumulate there. In previouslyknown plastic and rubber seals, the area of the doctor blade element isparticularly subject to color deposits because it is not possible forthe seal or sealing block to rest completely on the doctor blade elementfor different pressure forces leading to different curvatures of thedoctor blade element.

In another embodiment of the invention, a water spraying device moistensthe surface of the formatting plate facing the web with a fine jet ofwater, in order, particularly in the case of abrasive base papers, tolubricate the surfaces that rub together and thus minimize wear on theformatting plate. Tests have shown that the quantity of water needed forthis purpose is so small that no negative effects are found on the webedge. In this way, the serviceable life of the formatting plate can beextended from hours to up to several weeks. The formatting plate can beexchanged quickly using a clamping device, without requiring any specialadjustments to the coating edge limiter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a cross-sectional view, limited to the region essential tothe invention, of the coating edge limiter according to the presentinvention in a coating arrangement; and

FIG. 2 shows a top view of the coating edge limiter located on the rightin the coating arrangement.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Details of a coating arrangement are known and described in U.S. Pat.No. 4,250,211. For this reason, only those parts of such a coater thatrelate to the invention are described below.

A backing roll 1 around which a web 2 is guided, a barrier wall 4, adoctor blade element 3, and sealing blocks 5 located at both ends of thecoating arrangement form a coating chamber (not described in greaterdetail) into which color is fed under pressure. The doctor blade element3 extends over the entire width of the web and rests against a part 18.The application weight of the coating color is on the order of 5 to 16g/m² and is substantially determined by the pressure force of the doctorblade element 3. Excess color is guided away via the barrier wall 4.

According to FIG. 1, a formatting plate 8 almost completely covers anapplication zone 12 between the doctor blade element 3 and the barrierwall 4, so that an edge region 14 of the web 2 is sharply delineated andkept completely free of color, shown in FIG. 2. In each sealing block 5,there are three, or at least one, depression 11 in the form of a sink,which is open near the barrier wall 4 and ends before the doctor bladeelement 3. Each depression 11 extends parallel to the direction ofmovement of the paper web and is connected by a channel 13 to acentrally located water supply 20 in the sealing block 5. The watersupplied through the channels 13 forms a pressure cushion in thedepressions 11. The pressure cushion ensures that the formatting plate 8is pressed against the web 2 and reliably prevents the color which isunder pressure in the coating chamber, from emerging laterally through anarrow nip 15 having a width of approximately 1.5 mm, or between 1 mmand 3 mm. The water consumption of the coating edge limiter isapproximately 2 to 5 liters per minute. Compared to coating throughputof up to 1,000 liters per minute, this is negligibly small, therefore nospecial measures for water drainage are necessary. The water flowing outlaterally from the coating edge limiter is drained off together with theexcess color. The formatting plate 8 is arranged on a formatting plateholder 7 in such a way as to extend over the barrier wall 4 into a spacebetween the barrier wall 4 and the doctor blade element 3 in order tocompletely cover the application zone 12 in the edge region 14 of theweb 2. A clamping device 19 is used to attach the formatting plate 8 tothe formatting plate holder 7. The formatting plate holder 7 isconnected permanently to the sealing block 5 and extends outwardly overthe barrier wall 4 without touching the latter. On the free end of theformatting plate holder 7 is a water spraying device 9. A fine jet ofwater 16 is sprayed onto the formatting plate 8 at a pressure ofapproximately 0.5 bar to 1 bar through a needle nozzle 10 which has adiameter of about 0.5 mm.

With a sliding element 6, the coating edge limiter is arranged so as tobe movable as a unit at a right angle to the direction of web movement.

I claim:
 1. An apparatus for limiting a coating edge in a short dwellcoater comprising:a backing roll having a circumferential surface forguiding a web; a doctor blade element abutting said backing roll andextending laterally across a width of the web; a barrier wall orientedparallel to said doctor blade element, said barrier wall and said doctorblade element defining an application zone therebetween for applyingcoating to the web; two means for limiting the coating edge, each ofsaid edge limiting means comprising:a sealing block interposed betweensaid barrier wall and said doctor blade; a sliding element operativelyconnected to said sealing block and interposed between said barrier walland said doctor blade element; each of said coating edge limiting meansbeing arranged in an edge region of the web and movable in a directionperpendicular to a direction of movement of the web; a formatting plateholder having a free end and an opposite end connected to said sealingblock, said formatting plate holder extending outwardly over saidbarrier wall; and a thin formatting plate arranged on said formattingplate holder so as to extend over said barrier wall into a space betweensaid barrier wall and said doctor blade element and to cover theapplication zone in the edge region of the web; an upper surface of saidsealing block facing said formatting plate having at least onedepression extending parallel to the direction of movement of the weband connected to a water supply by a channel; said formatting plate andsaid sealing block defining therebetween a sealing gap having a width ofbetween 1 mm and 3 mm, the water introduced into the sealing gap throughthe depression producing a pressure cushion and pressing said formattingplate flatly against the web.
 2. The coating arrangement of claim 1,wherein said formatting plate is made of spring steel having a thicknessof less than or equal to 0.4 mm and is attached to said formatting plateholder by a clamping device.
 3. The coating arrangement of claim 2,wherein said formatting plate is movable in a direction perpendicular tothe direction of movement of the web and overlaps in width with a widthof said sealing block by approximately 5% to 20%.
 4. The coatingarrangement of claim 3, wherein said sealing block is made of metal. 5.The coating arrangement of claim 4, further comprising:a water sprayingdevice connected to the free end of said formatting plate holder, thewater spraying device having a needle nozzle for spraying a jet of wateronto said formatting plate.